Several active and formerly used federal facilities are faced with managing rapidly moving and expansive plumes of groundwater contaminated by explosives, particularly hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX). The Department of Defense currently has 583 sites with confirmed explosives-contaminated groundwater, and 88 additional sites are suspected of groundwater contamination with explosives and other organics. Due to health effects shown in humans, the U.S. Environmental Protection Agency (USEPA) has established a drinking water health advisory for RDX of 2 µg/L. At the time this project began, there was no generally accepted in situ process for remediation of RDX in groundwater. Available remediation alternatives were limited to long-term groundwater pumping and ex situ treatment followed by discharge or reinjection of treated water.
The U.S. Army Engineer Research and Development Center (ERDC) proposed using an in situ anaerobic bioremediation technique for the remediation of RDX contamination. ERDC conducted laboratory-scale studies to test the potential for anaerobic bioremediation by adding readily available carbon sources (electron donors) to create conditions in the subsurface conducive to the biological destruction of RDX, and other explosives compounds, by indigenous anaerobic microorganisms. This process was termed Biologically Active Zone Enhancement (BAZE). Laboratory testing of BAZE was sufficiently promising to warrant field testing using subsurface injections of sodium acetate. Since this project was initiated, in situ anaerobic bioremediation of RDX has been tested by other researchers and used at field scale, although validated cost and performance data still are not available.
The objectives of this demonstration were to test the BAZE technology under field conditions and to validate its potential to achieve regulatory cleanup criteria. The field demonstration was conducted at the former Nebraska Ordnance Plant (NOP) located in Mead, Nebraska. Sodium acetate was injected at 1- to 2-month intervals across an existing RDX-contaminated plume to create an in situ anaerobic biological treatment zone within the plume. The biologically active zone was sustained for 18 months and monitored throughout to develop the cost and performance data needed to transition this technology to potential users.
The project will involve bench- and field-scale assessments of anaerobic biostimulation of indigenous bacteria. The process involves the addition of acetate to induce anaerobic conditions in the aquifer as well as to serve as a nutrient source for the degradation of RDX. An initial treatability study will be performed using site soil and groundwater samples to determine the design parameters for use in the field-scale demonstration/validation project. The field demonstration will be completed using natural gradient groundwater flow to distribute the starch throughout the test plot following direct injection.
The BAZE system operated with ease for a year and a half. Induction of RDX degradation occurred at different times at the affected wells depending on the well’s distance from the injection site. Degradation was observed after 2 to 3 months at well MW-04, which was located 50 ft (15.2 m) downgradient of the treatment system, and after 12 months at well MW-10, located 200 ft (61 m) downgradient. Residual sodium acetate concentrations in the groundwater increased during the study, indicating sufficient levels were present to sustain treatment and support a microbial community. Biomass increased over the course of the demonstration, indicating biological stimulation, and oxidation-reduction potential (ORP) levels decreased from positive to negative levels, indicating anaerobic conditions. Together, the slow degradation induction, the residual acetate concentrations, increased biomass, and anaerobic conditions confirm the development of an enhanced microbial community that was responsible for the RDX degradation. RDX concentrations were reduced significantly, by up to 98%, during treatment, and the concentrations were maintained below 2 µg/L in some of the wells located closest to the injection points. The average concentration within the treatment zone at the end of treatment was near the regulatory limit (reduced from 66 to 14.6 µg/L in the closest well [15 ft downgradient] and from 191 to 7.1 µg/L in the wells located 30 ft downgradient). In summary, the results showed that it was possible to treat RDX to below 2 µg/L in a fully optimized system, and concentrations could be reliably reduced by 75% to more than 90% even during this small-scale demonstration.
The cost assessment indicated that BAZE could be implemented at full scale for considerably less than an ex situ pump-and-treat system. Other electron donors also could be used for enhancing in situ RDX biodegradation, and while no direct comparison is possible, it is significant that little to no biofouling was observed with sodium acetate injections in this demonstration. Biofouling is often a significant cost issue when adding electron donors.
The U.S. Army Corps of Engineers Kansas City District is the project lead on the Formerly Used Defense Site (FUDS) project and requires that remedial technologies: (1) adhere to local, state, and federal regulatory guidelines, (2) meet health advisory levels set forth in the ROD and by the EPA, (3) have no detrimental effect on overall water quality, (4) have no detrimental effect to the hydrodynamic characteristics of the aquifer, (5) have small surface footprint, (6) are simple to operate, and (7) have a low cost to performance ratio. Based on the results from this demonstration project, the BAZE system can meet these requirements, and the technology may be transitioned to the Kansas City District for implementation. The BAZE process does not produce any hazardous byproducts that need further disposal, as it is an extension of natural biodegradation. Impacts to secondary water quality parameters were temporary and limited to the treatment zone. The system is small and transportable and does not require any specialized equipment or custom-built prototypes.